The present invention relates to a thin film solar cell that has satisfactory photovoltaic conversion efficiency and a method for fabricating the thin solar cell.
The solar cell is attracting a great deal of attention as an alternative energy source substitute for fossil fuels such as petroleum, which are considered to be supplied less in future and have the problem of carbon dioxide emission as a cause of the global warming phenomenon.
The solar cell employs a pn junction in its photoelectric conversion layer for converting a light energy into an electric power, and silicon is generally most frequently employed as a semiconductor for constituting the pn junction. It is preferable to employ single crystal silicon in terms of photovoltaic conversion efficiency. However, the single crystal silicon has problems of material supply, areal increase, cost reduction and so on.
On the other hand, as a material advantageous for the achievement of areal increase and cost reduction, there is amorphous silicon. A thin film solar cell that employs this amorphous silicon as a photoelectric conversion layer has been put into practical use, however, its photovoltaic conversion efficiency is inferior to that of the single crystal silicon solar cell. Furthermore, the amorphous silicon causes a phenomenon called the Staebler-Wronski effect that the defect density in a film increases as light is applied, and therefore, the amorphous silicon solar cell is accompanied by the problem of deterioration with a lapse of time in terms of photovoltaic conversion efficiency.
Accordingly, in recent years, there have been conducted researches on the applications of polysilicon to the photoelectric conversion layer in order to provide a stabilized high photovoltaic conversion efficiency on the same level as that of the single crystal silicon solar cell and the areal increase and cost reduction on the same level as that of the amorphous silicon solar cell. In particular, a thin film polysilicon solar cell in which a thin film polysilicon is formed by means of a thin film forming technique by the chemical vapor deposition (CVD) method similar to that of the amorphous silicon is attracting a great deal of attention.
However, the current photovoltaic conversion efficiency of the thin film polysilicon solar cell fabricated by this method is merely on the same level as the photovoltaic conversion efficiency of the amorphous silicon solar cell. Several factors can be considered with regard to the low photovoltaic conversion efficiency, and one great factor is ascribed to the fact that the junction state at the interface between a doped layer and an intrinsic photoelectric conversion layer is not appropriately formed.
In the case of the aforementioned amorphous silicon solar cell, the state of the interface between a p-layer located on the incident light side and the intrinsic photoelectric conversion layer is particularly important. As a method for giving solution, the Japanese patent No. 2,846,639 discloses a method for providing a p/i interface layer in which carbon concentration is gradually varied between the p-layer constructed of a-SiC (amorphous silicon carbide) and the intrinsic photoelectric conversion layer constructed of a-Si (amorphous silicon). Japanese Patent Laid-Open Publication No. HEI 11-135814 discloses a method for setting a film forming rate of the intrinsic layer that has a thickness of several tens of nanometers and is put in contact with the p-layer slower than the film forming rate of the intrinsic layer to be subsequently formed. That is, these methods are the methods of providing an intermediate layer for improving the state of junction between the p-layer and the intrinsic layer.
Of course, applying the method of providing an intermediate layer to the thin film polysilicon solar cell contributes to the improvement of photovoltaic conversion efficiency. For example, Japanese Patent Laid-Open Publication No. HEI 11-135818 discloses a solar cell provided with a microcrystalline buffer layer that is formed by the plasma enhanced CVD method and provided between a p-type hydrogenated microcrystalline silicon layer and an intrinsic hydrogenated microcrystalline silicon layer. Damage of the p/i interface can be reduced by virtue of the existence of this microcrystalline buffer layer, and a open-circuit voltage and a fill factor value are improved to increase the photovoltaic conversion efficiency from 0.93% to 1.68%.
Generally, in the case of a polysilicon formed by the vapor deposition method such as the plasma enhanced CVD method, there is formed a phase of mixture including an amorphous component instead of the formation of a thin film that is completely made only of a crystal component. Then, in the amorphous component and a portion where the crystal component and the amorphous component adjoin each other, the bond state of silicon atoms is significantly disordered, and there is existing a great many uncombined hands, or the so-called dangling bond portions. The dangling bond forms a defect level in the forbidden band to consequently deteriorate the electric characteristics. Therefore, in the case of the thin film polysilicon solar cell, it is required to perform device design taking the state of existence of the crystal component and the amorphous component into due consideration. However, the method disclosed in Japanese Patent Laid-Open Publication No. HEI 11-135818 is no more than a method similar to the method for the solution of the aforementioned amorphous solar cell and is not regarded as a method that takes the existence of the crystal component into due consideration.
As a solar cell device design that takes the existence of the crystal component into consideration and has been disclosed so far, there are the methods disclosed in Japanese Patent Laid-Open Publication No. HEI 11-87742 and Japanese Patent Laid-Open Publication No. HEI 11-145498. The methods disclosed in these prior art reference documents are to obtain a photoelectric conversion layer that has a high crystallization ratio, a large crystal grain size and a firm crystal orientation property by providing an intrinsic amorphous silicon layer as a foundation layer of the intrinsic photoelectric conversion layer that includes a crystalline structure, controlling the crystallization ratio of the doped layer that includes a crystalline structure to be the foundation layer or taking similar measures. The structural characteristics of the thin film solar cells fabricated by these methods are as follows. In the case of the method disclosed in Japanese Patent Laid-Open Publication No. HEI 11-87742, an amorphous silicon layer is inserted between the doped layer that includes a crystalline structure and the intrinsic photoelectric conversion layer. In the case of the method disclosed in Japanese Patent Laid-Open Publication No. HEI 11-145498, the crystallization ratio of the doped layer, or the foundation layer of the photoelectric conversion layer is equal to or smaller than the crystallization ratio of the intrinsic photoelectric conversion layer.
However, the solar cell devices that take the existence of the crystal component into consideration and are disclosed in Japanese Patent Laid-Open Publication No. HEI 11-87742 and Japanese Patent Laid-Open Publication No. HEI 11-145498 have the problems as follows.
That is, the structures of the prior art solar cell devices are considered to be inappropriate for solar cells. The problems reside in the existence of a large amount of amorphous components in the doped layer itself or between the doped layer and the intrinsic photoelectric conversion layer. The problems owned by these structures are now described in detail below.
In a pin type thin film polysilicon solar cell constructed by successively stacking a p-type doped layer, an intrinsic photoelectric conversion layer and an n-type doped layer, separation of a pair of carriers (electron and hole) is performed by an internal electric field generated in the vicinity of a junction interface between the p-layer and the i-layer (intrinsic layer) or between the n-layer and the i-layer. If a large amount of amorphous components exists in the vicinity of the junction interface, then a large amount of defect levels exist due to the existence of a great many dangling bond portions in the amorphous component and the portion where the crystal component and the amorphous component adjoin each other, as described hereinabove. Consequently, the internal electric field is weakened to lower the open-circuit voltage. For example, considering the flow of carriers from the intrinsic layer to the n-layer, electrons flow from the intrinsic layer into the n-layer in the vicinity of the junction interface. However, in the structure where a large amount of amorphous components exists in the doped layer itself or between the doped layer and the intrinsic photoelectric conversion layer, a series resistance increases in a direction in which electrons flow, and consequently the fill factor is reduced. It is a matter of course that a similar phenomenon results in the junction interface between the p-layer and the intrinsic layer.
Due to the characteristic of the amorphous component having a great optical absorption coefficient on the shorter wavelength side, there is the problem that a considerable amount of incident light is disadvantageously absorbed in the doped layer itself when the doped layer includes a large amount of amorphous components and contributes nothing to photovoltaic conversion.
Conversely speaking, by designing the crystallization ratio of the doped layer equal to or greater than the crystallization ratio of the intrinsic layer, the existence of a large amount of amorphous components in the vicinity of the junction interface can be avoided. In addition, photoabsorption in the doped layer itself is also reduced by the increase in the crystallization ratio of the doped layer itself, and this allows a thin film solar cell having a high photovoltaic conversion efficiency to be fabricated.
With regard to the pin type thin film polysilicon solar cell, if a solar cell is constructed of an intrinsic layer and a doped layer determined by the single film characteristic values obtained by the normally used characteristic evaluation method such as electric conductivity measurement and light transmittance and reflectance measurement, then the characteristics of the solar cell are often inferior to the values expected by the aforementioned single film characteristic values. This is ascribed to the fact that the polysilicon thin film formed by the vapor deposition method such as the plasma enhanced CVD method often has an uneven microscopic structure in the direction of thickness. That is, in the pin type thin film polysilicon solar cell, the doped layer is usually formed so as to have a thickness of not greater than several tens of nanometers for the purposes of reducing the photoabsorption in the doped layer itself that contributes nothing to photoelectric conversion and reducing the series resistance component of the solar cell. However, in general, the formation conditions of the intrinsic layer and the doped layer are determined by evaluating the single film characteristics of each of the layers having about several hundreds to several thousands of nanometers. The film thickness of about several hundreds to several thousands of nanometers is the film thickness necessary for securing the reliability of the measurement values of a characteristic evaluation apparatus by a variety of optical methods or electrical methods. However, the thus-obtained characteristic values are no more than the information averaged throughout the entire film thickness of about several hundreds to several thousands of nanometers. Therefore, it is impossible to correctly estimate the state of the junction interface between the doped layer and the intrinsic layer, which have microscopic structures nonuniform in the direction of thickness. Accordingly, it is impossible to obtain solar cell characteristics on the same level as those expected by the single film characteristics when the layers are formed on the basis of the single film characteristics. Therefore, it is important to optimally design the structure of the junction interface between the doped layer and the intrinsic layer by using an evaluation method capable of correctly grasping a change in the microscopic structure in the direction of thickness of the doped layer and the intrinsic layer.
Accordingly, an object of the present invention is to provide a thin film solar cell that has an appropriate junction interface structure in which the crystallization ratio of a doped layer is equal to or greater than the crystallization ratio of an intrinsic layer as well as a method for fabricating the solar cell.
In order to achieve the aforementioned object, a first inventive aspect of the present invention provides a thin film solar cell wherein a p-type doped layer, an intrinsic photoelectric conversion layer and an n-type doped layer are stacked in this order or in order of the n-type doped layer, the intrinsic photoelectric conversion layer and the p-type doped layer and at least the doped layer formed firstly and the intrinsic photoelectric conversion layer formed secondly are silicon thin films including crystal components, the firstly formed doped layer having a crystallization ratio being equal to or greater than a crystallization ratio of the secondly formed intrinsic photoelectric conversion layer.
According to the above construction, the crystallization ratio of the firstly formed doped layer is equal to or greater than the crystallization ratio of the secondly formed intrinsic photoelectric conversion layer. Therefore, the amorphous silicon component does not increase in a direction from the secondly formed intrinsic photoelectric conversion layer to the firstly formed doped layer, and the structure of the junction interface between the doped layer and the intrinsic layer is optimized to allow a high photovoltaic convers-on efficiency to be obtained.
In this case, the aforementioned xe2x80x9csilicon thin film including the crystal componentxe2x80x9d means a silicon thin film in which the existence of a spectrum of about 520 cmxe2x88x921 corresponding to the crystal silicon is confirmed as the result of evaluation by the angle-lapping Raman scattering spectrometry method described in detail later.
In a variation of the first aspect of the present invention, the firstly formed doped layer is formed on a translucent substrate stacked with a transparent conductive film, and light enters from the firstly formed doped layer.
According to the above construction, light enters from the firstly formed doped layer having a high crystallization ratio and a small amount of amorphous silicon that causes a deterioration with a lapse of time of the photovoltaic conversion efficiency due to the aforementioned Staebler-Wronski effect, allowing a high photovoltaic conversion efficiency to be obtained.
A second aspect of the present invention provides a thin film solar cell wherein a p-type doped layer, an intrinsic photoelectric conversion layer and an n-type doped layer are stacked in this order or in order of the n-type doped layer, the intrinsic photoelectric conversion layer and the p-type doped layer and at least the intrinsic photoelectric conversion layer formed secondly and the doped layer formed thirdly are silicon thin films including crystal components, the thirdly formed doped layer having a crystallization ratio being equal to or greater than a crystallization ratio of the secondly formed intrinsic photoelectric conversion layer.
According to the above construction, the crystallization ratio of the thirdly formed doped layer is equal to or greater than the crystallization ratio of the secondly formed intrinsic photoelectric conversion layer. Therefore, the amorphous silicon component does not increase in a direction from the secondly formed intrinsic photoelectric conversion layer to the thirdly formed doped layer, and the structure of the junction interface between the doped layer and the intrinsic layer is optimized to allow a high photovoltaic conversion efficiency to be obtained.
In a variation of the second aspect of the present invention, light enters from the thirdly formed doped layer.
According to the above construction, light enters from the thirdly formed doped layer having a high crystallization ratio and a small amount of amorphous silicon that causes a deterioration with a lapse of time of the photovoltaic conversion efficiency, allowing a high photovoltaic conversion efficiency to be obtained.
A third aspect of the present invention further provides a thin film solar cell fabricating method for fabricating a thin film solar cell wherein a p-type doped layer, an intrinsic photoelectric conversion layer and an n-type doped layer are stacked in this order or in order of the n-type doped layer, the intrinsic photoelectric conversion layer and the p-type doped layer and at least the doped layer formed firstly and the intrinsic photoelectric conversion layer formed secondly are silicon thin films including crystal components, comprising the step of forming the first doped layer by a plasma enhanced chemical vapor deposition method using a VHF frequency band higher than a frequency of 13.56 MHz.
According to the above construction, the crystallization ratio of the first doped layer formed by the plasma enhanced CVD method using the VHF frequency band higher than the frequency of 13.56 MHz becomes equal to or greater than the crystallization ratio of the secondly formed intrinsic photoelectric conversion layer. Therefore, the amorphous silicon component does not increase in a direction from the secondly formed intrinsic photoelectric conversion layer to the firstly formed doped layer, and the structure of the junction interface between the doped layer and the intrinsic layer is optimized to allow a thin film solar cell having a high photovoltaic conversion efficiency to be formed.
A fourth aspect of the present invention provides a thin film solar cell fabricated by the thin film solar cell fabricating method set forth in the third aspect of the present invention.
According to the above construction, the crystallization ratio of the first doped layer formed by the plasma enhanced CVD method using the VHF frequency band higher than the frequency of 13.56 MHz becomes equal to or greater than the crystallization ratio of the secondly formed intrinsic photoeletric conversion layer. Therefore, the structure of the junction interface between the doped layer and the intrinsic layer is optimized to allow a high photovoltaic conversion efficiency to be obtained.
A fifth aspect of the present invention provides a thin film solar cell fabricating method for fabricating a thin film solar cell wherein a p-type doped layer, an intrinsic photoelectric conversion layer and an n-type doped layer are stacked in this order or in order of the n-type doped layer, the intrinsic photoelectric conversion layer and the p-type doped layer and at least the intrinsic photoelectric conversion layer formed secondly and the doped layer formed thirdly are silicon thin films including crystal components, comprising the step of forming the third doped layer by a plasma enhanced chemical vapor deposition method using a VHF frequency band higher than a frequency of 13.56 MHz.
According to the above construction, the crystallization ratio of the third doped layer formed by the plasma enhanced CVD method using the VHF frequency band higher than the frequency of 13.56 MHz becomes equal to or greater than the crystallization ratio of the secondly formed intrinsic photoelectric conversion layer. Therefore, the amorphous silicon component does not increase in a direction from the secondly formed intrinsic photoelectric conversion layer to the thirdly formed doped layer, and the structure of the junction interface between the doped layer and the intrinsic layer is optimized to allow a thin film solar cell having a high phctovoltaic conversion efficiency to be formed.
A sixth aspect of the present invention provides a thin film solar cell fabricated by the thin film solar cell fabricating method set forth in the fifth aspect of the present invention.
According to the above construction, the crystallization ratio of the third doped layer formed by the plasma enhanced CVD method using the VHF frequency band higher than the frequency of 13.56 MHz becomes equal to or greater than the crystallization ratio of the secondly formed intrinsic photoelectric conversion layer. Therefore, the structure of the junction interface between the doped layer and the intrinsic layer is optimized to allow a high photovoltaic conversion efficiency to be obtained.